PROJECT SUMMARY On a daily basis, each cell in our body is bombarded with some 30,000 endogenous DNA damages per day, the vast majority of which are repaired by Base Excision Repair (BER). The central hypothesis of this Program is that, understandably, defects in this BER process drive human carcinogenesis and affect responses to cancer treatments. The overall goal of this Program Project is to functionally characterize human genetic variation in the BER enzymes. To accomplish this, we are characterizing potentially damaging germline and tumor-associated SNPs in the B E R DNA glycosylases, and a number of the downstream enzymes in the BER pathway, using our strengths in bioinformatics, cell biology, biochemistry, structural biology and single molecule imaging in order to determine the functions of the wild-type and variant proteins. O u r p r e l i m i n a r y d a t a s u g g e s t that fundamental mechanistic studies are essential for interpreting human genetic variation and its influence on cancer etiology and tumor progression. Our program is informed and driven by the identification and characterization of germline and tumor-associated enzyme variants that may contribute to the altered DNA repair capacity of human BER enzymes. To realize our goals, variants in the BER genes are prioritized for study using bioinformatics (Core A) as well as enzymatic activity and structural information (Project 2). We then test for functional consequences of the variation using a powerful combination of biological, biochemical, structural, and single-molecule approaches. Core A provides the bioinformatics underpinning of the Program; Project 1, the biological studies of the human variant proteins in human cells; Project 2, the structure/function and biochemical analyses of the BER glycosylases; Project 3, the study of the BER repair process in chromatin; and Project 4, insights into the damage target search of the wild-type and variant BER proteins. Core B provides purified proteins to Projects 2-4 and cell cultures of variant clones to Projects 1-4, while Core C provides the administrative support. Taken together, the mechanistic results obtained by this Program Project provide a unique opportunity to underpin critical questions surrounding cancer etiology and cancer treatment, thus substantially impacting personalized medicine.